1. Field of the Invention
The present invention relates to a gas-liquid separation membrane provided with a plurality of gas-permeable pores that permit permeation of gas and prevent permeation of liquid by capillary force, and a production method thereof.
2. Related Background Art
A variety of proposals have been made heretofore on methods of utilizing gas-liquid selectivity of a membrane for separating a gas component or a liquid component from a state of gas-liquid mixed flow or gas-liquid two-phase flow. The gas-liquid separation membranes are generally categorized under two types, first and second types, which have their respective characteristics.
The gas-liquid separation membranes of the first type utilize gas permeability (diffusion) of membrane materials themselves and, specifically, they separate gas from liquid by the in-membrane diffusion difference between gas and liquid. Under this principle, however, the gas permeation speed is low, which raises various obstacles in practical use. One of countermeasures is a method of increasing the membrane area. The increase of the membrane area, however, results in increase in complexity and scale of the mechanism and increase in weight as well, which hinders decrease in scale and weight of apparatus using it.
The gas-liquid separation membranes of the second type utilize filter-like membranes provided with fine gas-permeable pores. In general, the liquid has surface tension and the capillary force appears in connection therewith. The capillary force acts stronger with decrease in the aperture size of a pore or a tube. Since the capillary force also acts at the gas-permeable pores, the membrane can prevent the permeation of liquid at the gas-permeable pores. On the other hand, the flow of gas is inhibited in inverse proportion to the cross-sectional area of the gas-permeable pores, but the gas is permitted to permeate. Therefore, the flow of gas is not interrupted, different from the liquid. As a result, the filter-like membrane with fine gas-permeable pores functions as a gas-liquid separation membrane. The capillary force is determined by the angle of contact of the liquid in the vicinity of the gas-permeable pores, and by the pore size (opening diameter) of the gas-permeable pores. When the pore sizes have some unevenness (dispersion), a force counterworking the pressure in separation of gas from liquid is theoretically dependent upon the largest pore size. In many conventional gas-liquid separation membranes the gas-permeable pores are connected to each other and these gas-permeable pores induce permeation of liquid. The gas-liquid separation membranes are also required to have the function of letting the gas smoothly permeate. The relation between the size of gas-permeable pores and the gas permeation resistance is generally expressed by the Hagen-Poiseuille formula. If the unevenness of pore sizes appears in the direction of smaller pore sizes of the gas-permeable pores, permeation efficiency of gas will be lowered. Namely, the gas-liquid separation membranes making use of the capillary force increase their performance and reliability with increase in the degree of evenness of pore sizes of the gas-permeable pores. In the case of the gas-liquid separation membranes as described, once the liquid passes through the interior of the gas-permeable pore for some reason, the capillary force will be no longer secured and the pore will permit the permeation of liquid, so as to cause leakage of the liquid, thereby damaging the function as a gas-liquid separation membrane.
As a solution to the problem in the gas-liquid separation membranes with the gas-permeable pores, Japanese Patent Application Laid-Open No. 07-124452 describes a method of making a porous film constituting the gas-liquid separation membrane, of a material with high water repellency. The porous film of this kind is obtained by making a film of a compression-molded product in which a liquid lubricant is preliminarily blended in particles of a raw material, thereafter removing the lubricant from the film, and stretching the film. This method, however, had a problem in structure, because it was difficult to make the pore sizes of the gas-permeable pores even by the method.
As a solution to this problem, Japanese Patent Application Laid-Open No. 05-214140 describes a technique of making films by compression molding of resin powder, thereafter bonding two or more films to each other, and stretching the bonded films, thereby making the pore sizes of the gas-permeable pores even.
It was, however, impossible to securely form independent gas-permeable pores by the technique as described in Japanese Patent Application Laid-Open No. 05-214140.
After all, none of the conventional methods succeeded in providing a gas-liquid separation membrane in which a plurality of gas-permeable pores were of even pore size and were independent of each other. The conventional methods also failed to solve the problem that, though the pore sizes of the gas-permeable pores were locally uniform, there occurred unevenness in the distribution of gas-permeable pores and the gas-permeable pores were connected to each other in a more macroscopic view. Therefore, the conventional methods have never permitted the gas-permeable pores to be formed independently of each other in uniform distribution and in even pore size. For this reason, the unevenness of pore sizes of the gas-permeable pores made it difficult to avoid entry of the liquid into the gas-permeable pores, and there was a limit to the performance and reliability of the gas-liquid separation membranes. It was also essentially impossible to form the gas-permeable pores with properties as desired.
An object of the present invention is to provide a gas-liquid separation membrane in which a plurality of gas-permeable pores are formed independently of each other in even pore size and in uniform distribution, while having high reliability and performance.
A gas-liquid separation membrane according to the present invention is a gas-liquid separation membrane comprising a plurality of gas-permeable pores that permit permeation of gas and that prevent permeation of liquid by capillary force, wherein the plurality of gas-permeable pores are formed independently of each other and in even pore size by laser machining.
A production method of a gas-liquid separation membrane according to the present invention is a method of producing a gas-liquid separation membrane having a plurality of gas-permeable pores that permit permeation of gas and that prevent permeation of liquid by capillary force, wherein the plurality of gas-permeable pores are formed independently of each other and in even pore size by laser machining.